Portable gas-measuring devices are usually used by persons who are located in areas in which they may be exposed to harmful gases. Such gas-measuring devices may be equipped with different models of sensors, e.g., infrared sensors, semiconductor sensors, electrochemical sensors, catalytic sensors or the like. However, the phenomenon of the so-called sensor poisoning plays a major role in respect to the ability of the gas sensor and hence of the gas-measuring device to function especially in the case of catalytic sensors.
Such catalytic sensors typically recognize not only a special gas, but there are so-called cross sensitivities, i.e., the sensor recognizes a plurality of different gases simultaneously, without being able to concretely indicate which of these gases it is actually measuring. However, the sensitivity to these gases may be different. The sensitivity to the recognized gases decreases in the course of the aging of such sensors. However, the sensitivity does not decrease uniformly for all recognized gases. The process is rather affected by the concrete history of the individual gas sensor, i.e., by the ambient and storage conditions and the gas species and quantities of gases to which the sensor was already exposed in the course of its life. However, the sensor may be damaged or poisoned above all by certain substances. Among others, substances such as silicon, sulfur compounds and polymerizing substances lead to such a poisoning in case of catalytic EX sensors (sensors that are used in an environment with potential explosion hazard and/or can detect explosible/explosive gases). One speaks of sensor poisoning in this connection if a certain gas or vapor to be detected cannot be processed in the catalytic sensor any longer and thus it cannot be detected any longer, or when the sensitivity is insufficiently low to initiate the necessary actions, e.g., alarms. The sensor in this case indicates a measurement result that is much too low compared to the actually present concentration of the gas to be detected. It may in this case happen that a sensor, which can measure a plurality of different gases, continues to correctly indicate a first gas, but it does not any longer correctly measure a second gas, for which it is already poisoned. This is especially significant if the calibrating gas, i.e., the gas with which the sensor sensitivity is set, corresponds to the gas for which the normal sensitivity was maintained. The loss of sensitivity for the second gas can thus remain suppressed.
Such a sensor poisoning may become especially relevant, for example, in case of catalytic sensors that shall detect combustible gases, e.g., in the area of firefighting, but also in the petrochemical industry, chemical industry or in mining. Such catalytic sensors can also recognize, besides methane, for example, different other gases and vapors, e.g., propane, pentane, butane or nonane and toluene or the like. For example, sensors that are used in the area of firefighting and shall indicate the presence of combustible gases or vapors, are often set by means of toluene or nonane, because they have a comparatively low sensitivity to these vapors. It is in this case assumed that a sensor thus set will adequately detect the entire range of combustible gases and vapors. However, it may be problematic that the sensitivity of the sensor to toluene and nonane is often retained for a long time, while the sensitivity to, e.g., methane declines much sooner, which is also called selective sensor poisoning for methane (or another corresponding gas). The gas sensor in this case measures the quantity of all gases present as a so-called sum signal, i.e., as a sum of a plurality of signals. The sum is composed of the individual measured values of the gases present. It cannot be recognized on the basis of the sum how high the actual percentage of the particular gases measured is. At the same time, the gas sensor is typically also unable to resolve the sum signal into the individual components. Thus, there may be a risk that the actually present concentrations of individual combustible gases, especially methane, but also of another gas, for which the sensor is already poisoned, is estimated incorrectly at an operating site. This may lead to drastic consequences for the user of the gas-measuring device in the worst case.
For example, US 2008/0257732A1, US 2006/0019402A1, and U.S. Pat. No. 5,670,115 A describe in this connection that sensor poisonings are determined by means of various calibrating or test gases, and the gas sensor to be tested is exposed to the calibrating or test gases one after another.
Test stations for gas-measuring devices, in which different test gases can be admitted simultaneously to a plurality of gas-measuring devices, are known from the documents JP 2006-003 115 A, U.S. Pat. No. 7,530,255 B2, and WO 2013/019178 A1.